Irrigation Water Conveyance and Application Methods
Subtopic: Design of irrigation channels and underground pipelines
Question Statement:
A parabolic shaped grassed waterway has top width 4 m, maximum depth 0.40 m, slope 2.5%, Manning's n = 0.035, no freeboard. The discharge carrying capacity of the waterway in m³ s⁻¹ is
Options:
(A) 1.38
(B) 1.52
(C) 1.76
(D) 1.96
Official Answer Key:
D
Step-by-Step Solution & Explanation
For a parabolic channel, cross-sectional area A = (2/3)Td = (2/3)(4)(0.4) = 1.067 m². Wetted perimeter P ≈ T + (8d²)/(3T) = 4 + (8×0.16)/(12) = 4.107 m. Hydraulic radius R = A/P = 0.2597 m. Manning: V = (1/n)R^(2/3)S^(1/2) = (1/0.035)(0.2597)^0.667(0.025)^0.5 = 1.839 m/s. Q = A×V = 1.067×1.839 = 1.96 m³/s.
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